An integrating sphere (or Ulbricht sphere) operates base on two principles: the diffraction or scattering of light inside a sphere and the diffusion of this light in such a way that it strikes certain light probing detectors.
The Functions Of The Integrating Sphere
The main function of an integrating sphere is to measure divergent light sources. As the name implies, it is spherical in shape (although several models are have a cuboid outer housing) and it is hollow. This construction is based, in part on the structure of base spheres. The sphere is internally coated with a highly reflective surface. Inside, it contains a variety of specialized equipment, such as baffles and detector ports. There is an entry opening for divergent light to enter whereupon approximately 99% of this light is diffracted throughout the sphere. This scatters, or diffracts, the light multiple times as if it were in a clear prism, causing each individual beam to reach the same intensity. A photo detector housed inside the sphere receives this intensity if it is struck by one of the beams and measures the proportional power (a sum of all ambient light inside the sphere) of the divergent light beam. The resulting radiance exiting the sphere extends for a full hemisphere while the irradiance striking the internal sphere wall is incident from a full hemisphere.
Integrating spheres must have a uniform reflective surface with a diffuse reflectivity that is at or near 100%. Proper calibration and configuration of the sphere and any related equipment is of the utmost importance in order to gain accurate, reliable readings. There are different reflective coatings that must be used at wavelengths (visible, infrared, and ultraviolet). Each coating must maintain the absence of fluorescence as the absorption of short-wavelength light by fluorescent materials can negatively affect the detector’s measurements.
The Uses Of The Integrating Sphere
The integrating sphere has several uses in photonics. Historically, it has been extensively used to measure luminous flux. The ongoing study of luminous flux has paved the way for common products such as LED and LCD backlights. Because of its properties as a uniform radiance source, the sphere can be used for imaging and nonimaging illumination. The sphere’s geometry allows it to capture and measure the power of laser diode beams. By increasing the number of ports and adjusting the sphere size, the sphere can take measurements from various incident angles. Measuring reflectance and transmittance of scattering materials remains the most prominent application of the integrating sphere. Similar to spectroscopy, the sphere’s readings can aid in classification or various materials. Products also have their place in radiometry. This is due to the spheres’ ability to collect all of a sample’s reflected radiation. Prior to the the introduction of the integrating sphere, it was nearly impossible to carry out field radiometry without getting erroneous results. Currently, integrating spheres have three main applications: light detectors (LSD), light sources (ISS), and integrating spheres for photometric material properties (ISMP). The internal configuration of structures and diffraction components varies between each sphere.
